PURPOSE: To dispense with the carrying of a partition wall into a garret after assembling the roof in a constructing site, and assemble the roof by use of a roof unit with the wall with few constructing processes.

CONSTITUTION: To fix a partition wall 3 to a roof panel 2, the base part 41 of a receiving tool 4 is first fixed to the common rafter 22 of the roof panel 2. A wood sash bar 31 fixed to the upper end part of each fire plate 33 constituting the wall 3 is firmly held by the support part 42 of the receiving tool 4. The fire plate 33 is also supported. Each fire plate 33 is fixed to the common rafter 22 by a fixture 5. A roof unit formed of a framework, the roof panel 2, the wall 3, and the receiving tool 4 is completed. A roof is assembled by this roof unit, and a garret is partitioned by the wall preliminarily fixed to the roof panel.